Polymeric aromatic carbamic acid esters (polyurethanes) such as diphenylmethane dicarbamates and the related higher homologs, polymethylene polyphenyl carbamates, have become increasingly important products particularly, for use in the preparation of the commercially valuable diphenylmethane diisocyanates and mixtures of diisocyanates and the polyisocyanates by the decomposition of such polymeric aromatic carbamic acid esters in a suitable solvent as shown in Rosenthal et al., U.S. Pat. Nos. 3,962,302 and 3,919,279.
A proposed prior art process for the preparation of polymeric aromatic carbamic acid esters (polyurethanes) is disclosed in Klauke et al, U.S. Pat. No. 2,946,768 and involves the condensation of aryl carbamic acid esters with carbonyl compounds in a dilute aqueous acid condensation medium. However, in such process the carbonyl compound such as formaldehyde tends to react at the nitrogen of the carbamate to produce along with desired polyurethanes, varying amounts, i.e., generally between 15 percent and 50 percent by weight, of undesirable (alkoxycarbonyl)-phenylaminomethylphenyl compounds which includes the various dimers, trimers, tetramers, etc. of such compounds (also referred to herein as "N-benzyl" compounds). Attempts to prepare mono or diisocyanates and polyisocyanates or to otherwise use the mixture containing the undesired N-benzyl compounds, which cannot be converted to an isocyanate by pyrolysis, and polyurethanes presents many problems. However, the undesired N-benzyl compounds may be catalytically rearranged to a desired polyurethane in accordance with the teachings of Shawl et al, U.S. Pat. No. 4,146,727. Accordingly, a product mixture from a condensation as disclosed in aforementioned U.S. Pat. No. 2,946,768 containing diurethanes and polyurethanes, N-benzyl compounds, unreacted alkylphenylcarbamates and other by-products such as amines may be contacted at temperatures of from abour 50.degree. C. to 170.degree. C. with a protonic acid medium having a strength at least equal to a 75 percent sulfuric acid such as concentrated sulfuric acid or an acid medium comprising a Lewis acid having a concentration of at least 0.5 percent by weight based on the total reaction mixture, while maintaining a minimum amount of water in the system, to catalytically convert or rearrange said N-benzyl compounds.
Shawl, U.S. Pat. No. 4,172,948, discloses a similar rearrangement of N-benzyl compounds may be achieved by use of anhydrous hydrogen chloride under super atmospheric pressure.
Condensation of aryl carbamic acid esters with formaldehyde may also be conducted with organic sulfonic acids. Shawl, U.S. Pat. No. 4,162,362, teaches that condensation in the presence of an organic sulfonic acid essentially eliminates formation of N-benzyl compounds and suppresses certain other undesirable side reactions.